1. Technical Field
This invention relates to chassis dynamometers, and more particularly to a frictionless chassis dynamometer incorporating concentrically disposed inner and outer roller bearings operable to cancel or mask frictional forces introduced by the bearings during operation of the dynamometer.
2. Discussion
Chassis dynamometers are used in a wide variety of applications, and particularly in connection with the testing of motor vehicle engine emissions pursuant to Environmental Protection Agency (EPA) emissions regulations for motor vehicles. Such dynamometers typically incorporate one or more "rolls" which are driven by one or more wheels of the vehicle under test. The rolls are typically coupled to an input shaft of some form of power absorption, or exchange, device which provides a controlled degree of rolling resistance to the rolls to simulate the road load and inertia forces normally encountered during vehicle operation.
The input shaft of the power absorption device and/or the rolls is typically supported by an annular roller bearing which is interposed between the input shaft of the power exchange device and a frame portion of the dynamometer. The bearing supports the input shaft and enables rotational movement of the input shaft relative to the fixed frame portion.
To compensate for the frictional forces introduced by the bearing on the input shaft, the dynamometer typically must be run for at least about twenty to thirty minutes to "warm-up" the bearings. It is then presumed that the friction of the bearings will vary with speed in accordance with known friction versus speed characteristics. These friction characteristics are stored in an external controller memory and then mathematically subtracted out from measurement data obtained by the dynamometer by an external controller by known friction compensation algorithms. The drawback with this approach, however, is that the friction of the bearings varies not only with speed but also with the temperature of the bearings, the loading on the bearings, and the duration of operating time intervals. These factors have proven difficult to accurately compensate for with correction algorithms. Moreover, friction compensation algorithms do not allow for variations in the degree of compensation being applied during an actual vehicle simulation test, only during execution of the compensation program itself. Accordingly, unaccounted for changes in vehicle loading or ambient room temperatures that occur during a vehicle simulation test would not be compensated for by conventional friction compensation algorithms.
Temperature, in particular, can prove especially difficult to determine and compensate for. More and more applications are requiring that dynamometers be operated in a cold testing facility (CTF) to enable carbon monoxide emissions to be measured at cold temperatures in accordance with EPA regulations. Since such "cold rooms" are also frequently used at significantly higher temperatures, the temperatures of the component parts of the dynamometer, and particularly the bearings, do not "settle" or become stable quickly. This is due in large part to the mass of the various components of the dynamometer, and the fact that many components, such as the rolls and bearings, are made from metal and thus form excellent paths for the transfer of heat and cold. Thus, while the ambient room temperature may stabilize quickly, the temperatures of the component parts of the dynamometer, and particularly the bearings, will not. Accordingly, bearing friction is particularly difficult, if not impossible, to accurately estimate and account for in test data, and stable bearings cannot be obtained until their temperatures stabilize.
Prior systems have attempted to alleviate this problem by providing means for heating the bearings such as by heated oil circulation systems or by closely regulating the temperature of the test cell in which the dynamometer is located. However, these systems have proven somewhat costly and unable to provide the degree of control necessary to accurately account for rapid ambient temperature changes.
With prior designs of chassis dynamometers, the bearings supporting the rolls and/or input shaft must also be selected with precision as a foremost consideration, rather than high durability. This usually means that high cost bearings must be used that introduce a minimum amount of friction, at the expense of lower cost bearings that have high durability but which introduce a greater amount of friction. Since higher precision, lower durability bearings are typically used, bearing maintenance and failure are higher than what would otherwise be experienced with high durability bearings. Moreover, periodic friction calibrations are typically required to compensate for variations in friction of the bearings resulting from wear.